Excitation Contraction Coupling

Question 1

What is excitation contraction coupling?

Answer 1

Linkage between excitation of muscle fibre membrane and onset of contraction

Question 2

What is action potential like in skeletal muscle?

Answer 2

Very fast - 10msec

Question 3

Tension response vs Action potential in skeletal muscle:

Answer 3

Latent period between both

Contraction of muscle doesn’t overlap with AP

Question 4

Where does latent period take place in skeletal muscle?

Answer 4

At peak of AP and start of contraction

Question 5

What happens during latent period?

Answer 5

Effect of changed membrane potential

Diffusion of signalling substance from extracellular space

Question 6

What effect does change in membrane potential do in latent period of skeletal muscle?

Answer 6

Electrical field change is limited to immediate vicinity (surrounding) of plasma membrane

Question 7

Why is the diffusion of signalling substance from extracellular space highly unlikely to cause latent period in skeletal muscle?

Answer 7

Time for diffusion much longer than latent period

Question 8

What does T-system enable?

Answer 8

Delivery of action potential deep into muscle fibre

Question 9

Where is calcium stored?

Answer 9

Sarcoplasmic reticulum

Question 10

What happens when action potential arrives at axon terminal in neuromuscular junction?

Answer 10

Acetylcholine diffuses across synaptic cleft and binds to nicotinic receptors

Question 11

What are nicotinic receptors like?

Answer 11

Ligand-gated channels

2 molecules of ACh bind to each receptor

Question 12

What happens when acetylcholine binds to nicotinic receptors?

Answer 12

They open and Na+ floods in (outweighs K+ moving out)

Cell depolarises

Question 13

What happens after depolarisation of motor end plate?

Answer 13

AP propagates along surface of muscle fibre - sarcolemma

AP propagates down T-tubule membrane

Question 14

What happens after action potential propagates down T-tubule?

Answer 14

Conformational change of DHP receptor

Question 15

What happens when DHP receptor changes shape?

Answer 15

Foot processes pull open RYR receptor (calcium release channel)

Question 16

What happens after RYR channel opens?

Answer 16

Calcium ions diffuse out of sarcoplasmic reticulum and flood into cytosol (down conc. gradient)

Question 17

What helps calcium release from sarcoplasmic reticulum?

Answer 17

Calcium-binding protein called calsequestrin

Question 18

What happens in latent period in skeletal muscle in regards to calcium?

Answer 18

There is an increase in intracellular calcium concentration

This causes muscle contraction

Question 19

How is calcium recycled in skeletal muscle?

Answer 19

Sarcoplasmic reticulum / terminal cisternae

Cytoplasm

Question 20

What are the two junctional foot proteins?

Answer 20

DHPR

RYR

Question 21

What does DHPR stand for?

Answer 21

Dihydropyridine receptor protein

Question 22

What type of receptor is DHPR?

Answer 22

L-type voltage-gated calcium channel

Question 23

Where is DHPR receptor found?

Answer 23

T-tubule membrane

Question 24

What does RYR stand for?

Answer 24

Ryanodine receptor protein

Question 25

What type of receptor is RYR?

Answer 25

Calcium release channel into sarcoplasmic reticulum

Question 26

What happens after calcium enters cytoplasm?

Answer 26

Binds to troponin C Allows strong actin-myosin binding

Question 27

What concentration does cytoplasmic calcium increase to?

Answer 27

10-7 M to 10-5 M

Question 28

What’s the key event resulting in interaction of actin and myosin filaments?

Answer 28

The increase in intracellular calcium concentration

Question 29

Example of dihydropyridine:

Answer 29

Nifedipine

Question 30

What muscle does Nifedipine address?

Answer 30

Smooth muscle

Question 31

What is nifedipine used to treat in smooth muscle?

Answer 31

Hypertension Migraine Atherosclerosis

Question 32

What is What is Nifedipine?

Answer 32

Voltage-gated calcium channel blocking drug

Question 33

Example of ryanodine:

Answer 33

Dantrolene

Question 34

What does Dantrolene do?

Answer 34

Spasmolytic drug acting as skeletal muscle relaxant Acts of ryanodine receptor

Question 35

What area in muscle does dantrolene address?

Answer 35

Sarcoplasmic reticulum

Question 36

What is dantrolene used to treat?

Answer 36

Muscle spasm Malignant hyperthermia

Question 37

What’s needed in order for relaxation of skeletal muscle?

Answer 37

Repolarisation Removal of calcium bac to resting levels

Question 38

What helps to return calcium back to resting levels?

Answer 38

Calcium ATPase (SERCA)

Question 39

What does SERCA stand for?

Answer 39

Sarcoplasmic endo plastic reticulum calcium ATPase

Question 40

What does SERCA do?

Answer 40

Forces calcium against concentration gradient

Question 41

What type of process is SERCA uptake?

Answer 41

Active process - need energy from hydrolysis of ATP

Question 42

What activates the SERCA pump in sarcoplasmic reticulum membrane?

Answer 42

Increase in intracellular calcium concentration

Question 43

How much calcium does SERCA actively transport?

Answer 43

2 calcium ions per molecule of ATP hydrolysed

Question 44

What does calcium concentration in cytoplasm decrease to after removed via SERCA?

Answer 44

Back to 10-7 M (resting levels)

Question 45

What is the role of calsequestrin?

Answer 45

Stores calcium at high concentrations in terminal cisternae

Question 46

What does the storing of calcium in calsequestrin result in?

Answer 46

Concentration gradient from sarcoplasmic reticulum to cytoplasm

Question 47

How many calcium ions bind to calsequestrin?

Answer 47

23 calcium ions per molecule

Question 48

Why don’t muscle cells in heart need an action potential to drive contraction?

Answer 48

Pacemaker cells - set electrical rhythm

Question 49

WHat are pacemaker cells?

Answer 49

Specialised muscle cells

Question 50

What is the resting potential like in pacemaker cells?

Answer 50

Unstable

Question 51

WHat do pacemaker cells undergo?

Answer 51

Automatic rhythmical depolarisation

Question 52

What is depolarisation of pacemaker potentials like?

Answer 52

They always depolarise to threshold Fast

Question 53

Graph of action potential in cardiac muscle:

Answer 53

Fast initial depolarisation Partial repolarisation and then plateau due to influx of calcium

Question 54

Duration of AP in cardiac muscle compared to in skeletal muscle:

Answer 54

Much longer - 350 ms

Question 55

How is change in tension different in cardiac muscle to skeletal muscle?

Answer 55

There is a change in tension during action potential - an overlap

Question 56

Where does 25% of required calcium for contraction go?

Answer 56

Enters through L-type calcium channels (DHPR) in T-tubule membrane

Question 57

What happens when 25% of calcium enters DHPR receptors in cardiac muscle?

Answer 57

Binds to RYR receptors + trigger calcium from stores Calcium released via RYR channels in sarcoplasmic reticulum

Question 58

How is relationship between DHR and RYR different in cardiac muscle to skeletal muscle?

Answer 58

No mechanical coupling It’s called calcium-induced calcium release

Question 59

Describe Calcium-indicted Calcium release:

Answer 59

25% Calcium enters DHPR channel Calcium binds to RYR receptor RYR receptor opens and 75% calcium released from sarcoplasmic reticulum

Question 60

What does relaxation of cardiac muscle require?

Answer 60

Decrease in cytoplasmic calcium concentration to 10-7 M

Question 61

What happens when cytoplasmic calcium concentration decreases?

Answer 61

Calcium ATPase in sarcoplasmic reticulum is activated

Question 62

What also helps remove calcium from cytoplasm?

Answer 62

Sodium:Calcium exchange in sarcolemma membrane Requires sodium ATPase